It was del Potro's first victory over Nadal since 2009 -- the year the former won his lone major at the U.S. Open -- and booked his spot at next month's year-end championships in London.

"I played so solid all the time, hitting the ball so hard," del Potro was quoted as saying by the ATP's official website. "I saw Rafa playing very far off the baseline, which is good for my game, for my serve, and confidence. That's the way to beat this guy."

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"Very few times I play against a player with a level like today. I really go home with the feeling that I didn't play bad. That's the thing. It is fair to say that. I am humble enough to say that I played well and I lost 6-2, 6-4."

Del Potro will likely have to play just as well or better to defeat Djokovic, who boosted his winning streak in China to 19 matches by downing Jo-Wilfried Tsonga 6-2 7-5 earlier Saturday.

Djokovic, now second in the rankings, regained his composure after a controversial line call in the second set.

"I would wish to get even a better performance tomorrow for the most important match this week," Djokovic, who crushed Nadal last week at the China Open, told reporters. "But we'll see."

When Djokovic and del Potro last met, Djokovic prevailed in a near five-hour classic in the Wimbledon semifinals.